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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 937 records · Page 52
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Assessment of sulfur trioxide formation due to enhanced interaction of nitrogen oxides and sulfur oxides in pressurized oxy-combustion

Pressurized oxy-combustion is emerging to be one of the best technologies for significantly decreasing the energy penalty for CO 2 capture in coal-fired power plants. However, the higher pressure boosts the formation of acid gases, including SO 3 and NO 2 , which could increase the risk of corrosion. The synergistic promotion of SO 3 and NO 2 formation in pressurized oxy-combustion is kinetically evaluated under representative conditions (1 ~ 30 atm, 600 ~ 1200 °C, NO/SO 2 = 0.1 ~ 5). We begin with a comprehensive mechanism (72 species and 428 reactions), covering nitrogen and sulfur chemistry, relying on GRI-Mech 3.0. This analysis shows that the interaction of SO X and NO X enhances the conversion rates of SO 2 → SO 3 , and this effect is more apparent at elevated pressures and lower temperatures. Mechanism analyses indicate that at elevated pressures, the formation pathways of SO 3 through HOSO 2 + O 2 = SO 3 + HO 2 , and NO 2 through HO 2 + NO = NO 2 + OH, are promoted due to the strong interaction between SO X and NO X . The intermediate between these two reactions is SO 2 + OH + M = HOSO 2 + M, resulting in a strong cycle, that can be expressed by the global reaction NO + SO 2 + O 2 = NO 2 + SO 3 . Finally, a nine-step reduced chemistry is developed and validated to accurately predict the formation of SO 3 in the post-flame region at elevated pressures.

42 ENGINEERING↗
NASA NTRS1973

The prospect of life on Jupiter.

We have simulated electrical discharges in the Jovian atmosphere, using anhydrous methane-ammonia mixtures, and shown the formation of simple aliphatic nitriles, amino-nitriles, and their oligomers. Including hydrogen sulfide in the gas mixture, it appears that sulfur-containing amino-nitriles are not formed, since the hydrolysate of the products did not contain the corresponding amino-acids. There is a strong analogy between these reactions and the classical spark reactions simulating the primitive earth's atmosphere. We are attempting a closer simulation of Jupiter's atmosphere by using appropriate temperature and pressure conditions. It seems that prebiotic synthesis on Jupiter may have reached an advanced state. As an alternative approach we have tested the survival ability of common terrestrial microorganisms in aqueous media at 102 atmospheres pressure and at 20 C in a simulated Jovian atmosphere. E. coli, S. marcescens, A. aerogenes, and B. subtilis will all tolerate 24 hr under these conditions with little death.

Ponnamperuma, C.↗
DOE OSTI2025

An unusual Co–S bond links B 12 chaperones in an interprotein complex

Chaperones in trafficking pathways ensure specificity of transition metal loading and protection against adventitious side reactions. In B 12 , an essential cofactor for humans, a cobalt ion is coordinated to a >1.3 kDa tetrapyrrolic scaffold, posing logistical challenges for its translocation. In this study, we report the 3.4 Å crystal structure of the human MMACHC and MMADHC B 12 chaperones, tethered via a rarely seen covalent cobalt–sulfur bond. B 12 is bound in the base-off state to MMACHC, with Cys-261 on MMADHC serving as the upper axial ligand; the lower-axial position is vacant. The propensity of thiolato-cobalamin derivatives bound to MMACHC to undergo spontaneous decomposition via general acid catalysis or reduction/oxidation chemistry is averted in the interprotein complex with MMADHC. An exposed face in the complex suggests an exit route for B 12 . No known clinical variants localize to the interprotein interface, consistent with the cobalt–sulfur bond being key to forming the high-affinity complex.

Science & Technology - Other Topics↗
NASA NTRS1994

Dimethylsulfide oxidation over the tropical South Atlantic: OH and other oxidants

The general course of events in the formation of a marine cloud begins with the emission of species which can eventually serve as nuclei around which water can condense to form a cloud droplet. In remote marine regions, cloud condensation nuclei (CCN) are primarily composed of sulfate, in either its acid or ammonium salt form. Most sulfate in these regions is the product of atmospheric oxidation of dimethyl sulfide (DMS), a reduced sulfur gas that is released by phytoplankton at the ocean surface. Therefore, in order to effectively quantify the links in the cloud-formation cycle, one must begin with a well-defined description of the atmospheric chemistry of DMS. The intent of this project has been to initiate development of a comprehensive model of the chemistry and dynamics responsible for the formation of clouds in the remote marine boundary layer. The primary tool in this work has been the Global/Regional Atmospheric Chemistry Event Simulator (GRACES), a global atmospheric chemistry model, which is under development within the Atmospheric Chemistry and Dynamics Branch of NASA-Ames Research Center. In this effort, GRACES was used to explore the first chemical link between DMS and sulfate by modeling the diurnal variation of DMS.

Hemming, Brooke L.↗
NASA NTRS1997

Redox Energy and Sulfur Chemistry in Prebiotic Polymer Synthesis and Replication

In the past year we have made significant progress in three research areas: (1) Most importantly, we discovered a new pathway of prebiotic amino acid synthesis in which formaldehyde and glycolaldehyde (substrates of the formose reaction) react with ammonia yielding alanine and homoserine in the presence of thiol catalysts. This thiol-dependent synthesis of amino acids undoubtedly occurs via amino acid thioester intermediates capable of forming peptides. This 'one-pot' reaction system operates under mild aqueous conditions, and like modern amino acid biosynthesis, uses sugar intermediates which are converted to amino acids by energy-yielding redox disproportionation. (2) Finally, in preparation for the analysis of Martian meteorite samples, we upgraded our HPLC system and developed an improved method capable of detecting a I femtomole of amino acid enantiomers. (3) We completed our analysis of the energetics of metabolism that revealed that life depends on biosynthetic processes driven by chemical energy made available by the redox disproportionation of carbon groups of sugars. We established that the favorable energy of redox disproportionation is based on the universal reduction potentials of carbon groups. We concluded that it is hard to imagine any other organic molecule besides sugars (formaldehyde oligomers) having the energy and reactivity needed to drive either modem biosynthesis or the chemical processes behind its origin.

Weber, Arthur L.↗
DOE OSTI2020

Synergistic effect of dual Brønsted acidic deep eutectic solvents for oxidative desulfurization of diesel fuel

Deep eutectic solvents (DESs) are generally used as attractive solvents or extractants rather than catalysts in the field of synthesis and separation. Herein, dual Brønsted acidic DESs were prepared by mixing sulfonated quaternary ammonium and oxalic acid as extractants and catalysts for deep oxidative desulfurization. Compared to the DESs with single acidic group, the dual acidic DESs exhibited much higher catalytic activity likely due to the synergistic effect. The 1 H NMR study indicated that the weak interaction between DES and sulfur compounds led to a low extraction capacity. However, the sulfur removal could reach 97.7% under optimized reaction conditions. Density functional theory (DFT) calculations demonstrated that sulfonic group and carboxylic group are of equal importance to desulfurization performance. Further, the desulfurization efficiency was also strongly affected by the viscosity of DESs. Here, we put forward a new strategy that the reaction time could be sharply shortened to 40 min and 100 min by adding low viscous acetonitrile and polyethylene glycol into DES to form mixing reaction system, respectively.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Structural and Biophysical Properties of a [4Fe-4S] Ferredoxin-Like Protein from Synechocystis sp. PCC 6803 with a Unique Two Domain Structure

Electron carrier proteins (ECPs), binding iron-sulfur clusters, are vital components within the intricate network of metabolic and photosynthetic reactions. They play a crucial role in the distribution of reducing equivalents. In Synechocystis sp. PCC 6803, the ECP network includes at least nine ferredoxins. Previous research, including global expression analyses and protein binding studies, has offered initial insights into the functional roles of individual ferredoxins within this network. This study primarily focuses on Ferredoxin 9 (slr2059). Through sequence analysis and computational modeling, Ferredoxin 9 emerges as a unique ECP with a distinctive two-domain architecture. It consists of a C-terminal iron-sulfur binding domain and an N-terminal domain with homology to Nil-domain proteins, connected by a structurally rigid 4-amino acid linker. Notably, in contrast to canonical [2Fe-2S] ferredoxins exemplified by PetF (ssl0020), which feature highly acidic surfaces facilitating electron transfer with photosystem I reaction centers, models of Ferredoxin 9 reveal a more neutral to basic protein surface. Using a combination of electron paramagnetic resonance spectroscopy and square-wave voltammetry on heterologously produced Ferredoxin 9, this study demonstrates that the protein coordinates 2x[4Fe-4S]2+/1+ redox-active and magnetically interacting clusters, with measured redox potentials of -420 +/- 9 mV and -516 +/- 10 mV vs SHE. A more in-depth analysis of Fdx9's unique structure and protein sequence suggests that this type of Nil-2[4Fe-4S] multi-domain ferredoxin is well conserved in cyanobacteria, bearing structural similarities to proteins involved in homocysteine synthesis in methanogens.

cyanobacteria↗
NASA NTRS1995

Biospheric effects of a large extraterrestrial impact: Case study of the cretaceous/tertiary boundary crater

The Chicxulub impact crater, buried in the Yucatan carbonate platform in Mexico, is the site of the impact purported to have caused mass extinctions at the Cretaceous/Tertiary (K/T) boundary. A recently discovered Chicxulub ejecta deposit in Belize contains evidence of carbonate vaporization and precipitation from the vapor plume. Sulfate clasts are almost absent in the Belize ejecta, but are abundant in the coarse ejecta near the crater rim, hwich may reflect the greater abundance of sulfates deep in the target section. The absence of sulfate precipitates in Belize may indicate that most of the vaporized sulfur was deposited in the upper atmosphere. Hydrocode modeling of the impact indicates that between 0.4 to 7.0 x 10(exp 17) g of sulfur were vaporized by the impact in sulfates. Laser experiments indicate that SO2, SO3, and SO4 are produced, and that complex chemical reactions between plume constituents occur during condensation. The sulfur released as SO3 or SO4 converted rapidly into H2HO4 aerosol. A radiative transfer model coupled with a model of coagulation predicts that the aerosol prolonged the initial blackout period caused by impact dust only if it contained impurities. The sulfur released as SO2 converted to aerosol slowly due to the rate limiting oxidation of SO2. Radiative transfer calculations combined with rates of acid production, coagulation, and diffusion indicate that solar transmission was reduced to 10-20 percent of normal for a period of 8-13 years. This reduction produced a climate forcing (cooling) of -300 Wm(exp -2), which far exceeded the +8 Wm(exp -2) greenhouse warming caused by the CO2 released through the vaporization of carbonates, and therefore produced a decade of freezing and near-freezing temperatures. Several decades of moderate warming followed the decade of severe cooling due to the long residence time of CO2. The prolonged impact winter may have been a major cause of the K/T extinctions.

Pope, Kevin O.↗
NASA NTRS2006

Moessbauer Mineralogical Evidence for Aqueous Processes at Gusev Crater and Meridiani Planum

The Moessbauer spectrometers on the MER rovers have measured the relative abundances of iron with respect to both oxidation state and iron-bearing phase at Gusev Crater (Spirit rover) and Meridiani Planum (Opportunity rover). The assemblage of phases indicates aqueous alteration processes at both landing sites. Although the rock and soil of the Gusev Crater plains are dominated by Fe(2+) in olivine-bearing basalt (approx.Fo60), a Fe(3+)-rich component (nanophase ferric oxide, np-Ox) has significant abundance in surface soils (13-28% of total Fe) and in the surface coatings (rinds) of certain rocks (39%) but not in rock interiors exposed by grinding (5-6%). The mode of occurrence of np-Ox implies that it is the product of oxidative alteration of Fe(2+) silicate and oxide phases in the presence of H2O. The ubiquitous presence of sulfur in soil and in rock coatings, as determined by the MER-A APXS instrument, suggests that the alteration occurred under acid-sulfate conditions, so that both hydrolytic and sulfatic reactions are viable. A possible source for the weathering agents is volcanic emanations rich in H2O and SO2. Generally, rocks in the Columbia Hills are significantly more altered than those in the Gusev plains, with a higher proportion of Fe(3+) oxide phases compared to Fe(2+) silicate phases. This mineralogical dichotomy implies a difference in the timing, rate, duration, and/or mechanism of alteration for basaltic material in the Gusev plains compared to basaltic material in the Columbia Hills. It is possible, for example, that the basaltic material in the Columbia Hills underwent aqueous alteration in a paleoclimate that favored nearly complete alteration and that the basaltic material of the Gusev plains will not achieve the degree of alteration exhibited by the Columbia Hills under current martian surface conditions.

Morris, R. V.↗
NASA NTRS1987

Atmospheric H2O2 measurement: comparison of cold trap method with impinger bubbling method

Collection of atmospheric H2O2 was performed by a cold trap method using dry ice-acetone as the refrigerant. The air was drawn by a pump into a glass gas trap immersed in the dry ice-acetone slush in a dewar flask at a flow rate of 2.5 l min-1 for approximately 2 h. Collection efficiency was > 99% and negligible interferences by O3, SO2 or organic matter with the collected H2O2 in the trap were observed. This method was compared with the air impinger bubbling method which has been previously described (Kok et al., 1978a, b, Envir. Sci. Technol. 12, 1072-1080). The measured total peroxide (H2O2 + organic peroxide) values in a series of aim samples collected by the impinger bubbling method (0.06-3.7 ppb) were always higher than those obtained by the cold trap method (0.02-1.2 ppb). Laboratory experiments suggest that the difference in values between the two methods probably results from the aqueous phase generation of H2O2 and organic peroxide in the impinger solution by a reaction of atmospheric O3 with olefinic and aromatic compounds. If these O3-organic compound reactions which occur in the impinger also occur in aqueous droplets in the atmosphere, the process could be very important for aqueous phase generation of H2O2 in clouds and rainwater.

NASA Discipline Exobiology↗
DOE OSTI2025

Impact of SO 2 on NiFe Nanoparticle Exsolution and Dissolution from LaFe 0.9 Ni 0.1 O 3 Perovskite Oxides

Ni-doped LaFeO 3 perovskite oxide is a promising cathode material for solid oxide electrolysis cells (SOECs) designed for CO 2 /H 2 O coelectrolysis. Here, the performance of LaFe 0.9 Ni 0.1 O 3 is being investigated under real-world conditions that include exposure to acid gases, such as SO 2 , relevant to SOEC operation. Experiments show that LaFe 0.9 Ni 0.1 O 3 exsolves NiFe nanoparticles, along with the formation of surface SO 4 2– and SO 3 2– after being exposed to 200 ppm of SO 2 . This suggests that the ionic diffusion of Ni 3+ and Fe 3+ between the bulk and the surface remains unaffected throughout the exsolution–dissolution–exsolution cycle. Thermochemical water splitting has been employed as a probe reaction to evaluate the catalytic properties of the exsolved NiFe nanoparticles. These nanoparticles demonstrated improved hydrogen production compared to bare perovskite oxide substrates. However, after exposure to SO 2 , the formation of Fe-rich NiFe nanoparticles led to poor thermocatalytic performance and rapid deactivation of the perovskite at elevated temperatures. Density functional theory (DFT) analysis was utilized to validate the experimental findings, indicating a significantly negative reaction energy for water splitting over exsolved Fe, as well as stronger binding of SO 2 to Fe than to Ni. Computational analysis further suggests that the presence of surface sulfate promotes the formation of Fe-rich NiFe nanoparticles, aligning with the experimental results. Overall, this study clarifies how SO 2 affects the structure of SOEC perovskite oxide candidate materials. Future engineering efforts should focus on enhancing nanoparticle exsolution and sulfur resistance, which is crucial for improving the hydrogen production capacity of La-based perovskite oxides for electro- and thermocatalytic water splitting in real environments containing acid gases.

Najimu, Musa [Univ. of Southern California, Los An↗
NASA NTRS1990

A reverse KREBS cycle in photosynthesis: consensus at last

The Krebs cycle (citric acid or tricarboxylic acid cycle), the final common pathway in aerobic metabolism for the oxidation of carbohydrates, fatty acids and amino acids, is known to be irreversible. It liberates CO2 and generates NADH whose aerobic oxidation yields ATP but it does not operate in reverse as a biosynthetic pathway for CO2 assimilation. In 1966, our laboratory described a cyclic pathway for CO2 assimilation (Evans, Buchanan and Arnon 1966) that was unusual in two respects: (i) it provided the first instance of an obligate photoautotroph that assimilated CO2 by a pathway different from Calvin's reductive pentose phosphate cycle (Calvin 1962) and (ii) in its overall effect the new cycle was a reversal of the Krebs cycle. Named the 'reductive carboxylic acid cycle' (sometimes also called the reductive tricarboxylic acid cycle) the new cycle appeared to be the sole CO2 assimilation pathway in Chlorobium thiosulfatophilum (Evans et al. 1966) (now known as Chlorobium limicola forma thiosulfatophilum). Chlorobium is a photosynthetic green sulfur bacterium that grows anaerobically in an inorganic medium with sulfide and thiosulfate as electron donors and CO2 as an obligatory carbon source. In the ensuing years, the new cycle was viewed with skepticism. Not only was it in conflict with the prevailing doctrine that the 'one important property ... shared by all (our emphasis) autotrophic species is the assimilation of CO2 via the Calvin cycle' (McFadden 1973) but also some of its experimental underpinnings were challenged. It is only now that in the words of one of its early skeptics (Tabita 1988) 'a long and tortuous controversy' has ended with general acceptance of the reductive carboxylic acid cycle as a photosynthetic CO2 assimilation pathway distinct from the pentose cycle. (Henceforth, to minimize repetitiveness, the reductive pentose phosphate cycle will often be referred to as the pentose cycle and the reductive carboxylic acid cycle as the carboxylic acid cycle.) Aside from photosynthetic pathways which are the focus of this article, CO2 assimilation is also known to sustain autotrophic growth via the acetyl-CoA pathway (Wood et al. 1986). Our aim here is to discuss (i) the findings that led our group to the discovery of the reductive carboxylic acid cycle, (ii) the nature and resolution of the controversy that followed, and (iii) the possible evolutionary implications of the cycle as an ancient mechanism for photosynthetic CO2 assimilation that preceded the pentose cycle and served as a precursor of the Krebs cycle in aerobic metabolism.

Non-NASA Center↗
DOE OSTI2023

Organic Matrix Derived from Host–Microbe Interplay Contributes to Pathological Renal Biomineralization

Matrix stones are a rare form of kidney stones. They feature a high percentage of hydrogel-like organic matter, and their formation is closely associated with urinary tract infections. Herein, comprehensive materials and biochemical approaches were taken to map the organic-inorganic interface and gather insights into the host-microbe interplay in pathological renal biomineralization. Surgically extracted soft and slimy matrix stones were examined using micro-X-ray computed tomography and various microspectroscopy techniques. Higher-mineral-density laminae were positive for calcium-bound Alizarin red. Lower-mineral-density laminae revealed periodic acid-Schiff-positive organic filamentous networks of varied thickness. These organic filamentous networks, which featured a high polysaccharide content, were enriched with zinc, carbon, and sulfur elements. Neutrophil extracellular traps (NETs) along with immune response-related proteins, including calprotectin, myeloperoxidase, CD63, and CD86, also were identified in the filamentous networks. Expressions of NETs and upregulation of polysaccharide-rich mucin secretion are proposed as a part of the host immune defense to "trap" pathogens. These host-microbe derived organic matrices can facilitate heterogeneous nucleation and precipitation of inorganic particulates, resulting in macroscale aggregates known as "matrix stones". These insights into the plausible aggregation of constituents through host-microbe interplay underscore the unique "double-edged sword" effect of the host immune response to pathogens and the resulting renal biominerals.

59 BASIC BIOLOGICAL SCIENCES↗
NASA NTRS1975

On the origin of molecular 'handedness' in living systems

Elementary particle effects (beta-decay) provide only a weakly handed radiation in the biologically effective energy ranges. Global magnetic effects coupled to sunlight are randomized by paleomagnetic reversals. A persistent terrestrial handed bias at possible local biopoetic sites offers a more promising explanation for the origin of the 'handedness' of the molecules found among living systems on earth. Magnetite in lava flows maintains a handed bias for surface catalysis through many magnetic reversals. Indirect evidence for the hypothesis that magnetite contaminated with sulfur is a possible biopoetic site (Granick, 1957) has been provided by the molecular structure of ferredoxin - a single strand of the 14 primordial amino acids wrapped around an FeS core. Lava flows have been suggested as biopoetic sites by Fox (1964), since their temperature and chemical composition might allow for the rapid synthesis of prebiotic compounds at the surface of the primitive earth. The additional fact that magetite in lava flows also provides a persistent handed site for surface catalysis offers a further argument for the experimental investigation of this specific biopoetic environment.

Noyes, H. P.↗
DOE OSTI2020

Model bias in simulating major chemical components of PM 2.5 in China

High concentrations of PM 2.5 (particulate matter with an aerodynamic diameter less than 2.5 µm) in China have caused severe visibility degradation. Accurate simulations of PM 2.5 and its chemical components are essential for evaluating the effectiveness of pollution control strategies and the health and climate impacts of air pollution. In this study, we compared the GEOS-Chem model simulations with comprehensive datasets for organic aerosol (OA), sulfate, nitrate, and ammonium in China. Model results are evaluated spatially and temporally against observations. The new OA scheme with a simplified secondary organic aerosol (SOA) parameterization significantly improves the OA simulations in polluted urban areas, highlighting the important contributions of anthropogenic SOA from semivolatile and intermediate-volatility organic compounds. The model underestimates sulfate and overestimates nitrate for most of the sites throughout the year. More significant underestimation of sulfate occurs in winter, while the overestimation of nitrate is extremely large in summer. The model is unable to capture some of the main features in the diurnal pattern of the PM 2.5 chemical components, suggesting inaccuracies in the presented processes. Potential model adjustments that may lead to a better representation of the boundary layer height, the precursor emissions, hydroxyl radical concentrations, the heterogeneous formation of sulfate and nitrate, and the wet deposition of nitric acid and nitrate have been tested in the sensitivity analysis. The results show that uncertainties in chemistry perhaps dominate the model biases. The proper implementation of heterogeneous sulfate formation and the good estimates of the concentrations of sulfur dioxide, hydroxyl radical, and aerosol liquid water are essential for the improvement of the sulfate simulation. The update of the heterogeneous uptake coefficient of nitrogen dioxide significantly reduces the modeled concentrations of nitrate. However, the large overestimation of nitrate concentrations remains in summer for all tested cases. The possible bias in the chemical production and the wet deposition of nitrate cannot fully explain the model overestimation of nitrate, suggesting issues related to the atmospheric removal of nitric acid and nitrate. A better understanding of the atmospheric nitrogen budget, in particular, the role of the photolysis of particulate nitrate, is needed for future model developments. Moreover, the results suggest that the remaining underestimation of OA in the model is associated with the underrepresented production of SOA.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1987

The green non-sulfur bacteria: a deep branching in the eubacterial line of descent

Ribosomal RNA sequence comparisons define a phylogenetic grouping, the green non-sulfur bacteria and relatives (GNS), known to contain the genera Chloroflexus, Herpetosiphon and Thermomicrobium--organisms that have little phenotypic similarity. The unit is phylogenetically deep, but entirely distinct from any other eubacterial division (phylum). It is also relatively ancient--branching from the common eubacterial stem earlier than any other group of eubacteria reported thus far. The group phenotype is predominantly thermophilic, and its thermophilic members, especially Thermomicrobium, are more slowly evolving than Herpetosiphon, a mesophile. The GNS unit appears significantly older than either the green sulfur bacteria or the cyanobacteria--making it likely that organisms such as Chloroflexus, not the cyanobacteria, generated the oldest stromatolites, which formed over three billion years ago.

Non-NASA Center↗
DOE OSTI2020

New perspectives on butyrate assimilation in Rhodospirillum rubrum S1H under photoheterotrophic conditions

Background: The great metabolic versatility of the purple non-sulfur bacteria is of particular interest in green technology. Rhodospirillum rubrum S1H is an α-proteobacterium that is capable of photoheterotrophic assimilation of volatile fatty acids (VFAs). Butyrate is one of the most abundant VFAs produced during fermentative biodegradation of crude organic wastes in various applications. While there is a growing understanding of the photoassimilation of acetate, another abundantly produced VFA, the mechanisms involved in the photoheterotrophic metabolism of butyrate remain poorly studied. Results: In this work, we used proteomic and functional genomic analyses to determine potential metabolic pathways involved in the photoassimilation of butyrate. We propose that a fraction of butyrate is converted to acetyl-CoA, a reaction shared with polyhydroxybutyrate metabolism, while the other fraction supplies the ethylmalonyl-CoA (EMC) pathway used as an anaplerotic pathway to replenish the TCA cycle. Surprisingly, we also highlighted a potential assimilation pathway, through isoleucine synthesis and degradation, allowing the conversion of acetyl-CoA to propionyl-CoA. We tentatively named this pathway the methylbutanoyl-CoA pathway (MBC). An increase in isoleucine abundance was observed during the early growth phase under butyrate condition. Nevertheless, while the EMC and MBC pathways appeared to be concomitantly used, a genome-wide mutant fitness assay highlighted the EMC pathway as the only pathway strictly required for the assimilation of butyrate. Conclusion: Photoheterotrophic growth of Rs. rubrum with butyrate as sole carbon source requires a functional EMC pathway. In addition, a new assimilation pathway involving isoleucine synthesis and degradation, named the methylbutanoyl-CoA (MBC) pathway, could also be involved in the assimilation of this volatile fatty acid by Rs. rubrum.

59 BASIC BIOLOGICAL SCIENCES↗